In electrically steerable parasitic array radiator (ESPAR) antenna systems, the radiation direction of an antenna can be controlled. In this regard, the radiation direction of an antenna refers to a direction in which the antenna transmits or receives electromagnetic waves.
An ESPAR antenna system includes a monopole antenna and parasitic elements which are vertically mounted in proximity to the monopole antenna on a prepared ground surface. Each of the parasitic elements includes a lumped element having a variable reactance, and the radiation direction of the monopole antenna is controlled by adjusting the reactance of each lumped element. Since parasitic elements of the ESPAR antenna system are vertically mounted on the ground surface and lumped elements are included in such parasitic elements, only the radiation direction of the monopole antenna in the ESPAR antenna system parallel to the ground surface on which the monopole antenna is mounted can be changed, and a radiation direction perpendicular to the ground surface cannot be changed.
Thus, ESPAR antenna systems have a limitation in terms of controlling the radiation direction of an antenna.